USPTO Patents Application 10792270

per

WORLD INTELLECTUAL PROPERTY ORGANIZATION
International Bureau

INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)

(51) International Patent Classification 4
B27N 3/02, B32B 21/02

Al

(11) International Publication Number:
(43) International Publication Date:

WO 89/ 08539

21 September 1989 (21.09.89)

(21) International Application Number: PCT/SE89/O013O

(22) International Filing Date: 13 March 1989 (13.03.89)

(31) Priority Application Number: 8800950-1

(32) Priority Date: 16 March 1988 (16.03.88)

(33) Priority Country: SE

(71) Applicant (for all designated States except US): INSTI-

TUTET F6R TRATEKNISK FORSKNING [SE/
SE]; Box 5609, S-l 14 86 Stockholm (SE).

(72) Inventor; and

(75) Inventor/Applicant (for US only) : BLtJMER, Hartwig
[DE/SE]; Seldonsvagen 15, S-l 75 39 Jarfalla (SE).

(74) Agents: HJARNE, Per-Urban et aL; H. Albihns Patent-
byra AB, Box 3137, S-l 03 62 Stockholm (SE).

(81) Designated States: AT (European patent), AU. BE (Eu-
ropean patent), CH (European patent), DE (Euro-
pean patent), FI, FR (European patent), GB (Euro-
pean patent), IT (European patent), JP, LU (Euro- j
pean patent), NL (European patent), SE (European '

Published

With international search report.

(54)™.= »g™^™™<™g™* OF CHIPBOARD, AND CHIPBOARD MANUFACTURED IN

(57) Abstract

and omiSnVS t0 a h met rK d ^ manu J actu u re of chipboard comprising a centre layer (18) of coarse chios
n?l?n^c^ k ( i° f ^ !PS W ^°- Se fi u re - S Sr . e ° nented m the plane of the board " In order "> be able to reduce the densrtv
WnrSn r5!?S,° ar f- Whlle r 5 tamm S m * msic characteristic profile of the board, it is proposed in accordance with th'e
invention that the chips used to compose the centre layer (18) will comprise end-grain cut, flake-like chips (22) whose fi-
bres are oriented in the thickness direction (24) of the chips, the chip fibres in the centre layer being Torien?ed subsCtiallv
m«hod * t0 P ° f thC Chipb6ard - TCg in_Vention also relate * to chipboard manured TS^^^

FOR THE PURPOSES OF INFORMATION ONLY

cations un'de^^^

Austria

Australia

Barbados

Belgium

Bulgaria

Benin

Brazil

Central African Republic

Congo

Switzerland

Cameroon

Germany, Federal Republic of
Denmark

FR

France

GA

Gabon

GB

United Kingdom

HU

Hungary

rr

Italy

jp

Japan

KP

Democratic People's Republic

of Korea

KR

Republic of Korea

LI

Liechtenstein

LK

Sri Lanka

LU

Luxembourg

MC

Monaco

ML Mali

MR Mauritania

MW Malawi

NL Netherlands

NO Norway

RO Romania

SD Sudan

SE Sweden

SN Senegal

SU Soviet Union

TO Chad

TG Togo

r

WO 89/08539

PCT/SE89/00130

1

A method for the manufacture of c hipboard , and chipboard
manufactured i n accordance with the method.

The present invention relates to a method for the manu-
5 facture of chipboard, or particle board, comprising a

centre layer of coarse chips and outer layers of chips
whose fibres are positioned in the plane of the board.

Conventional chipboard comprised of a centre layer of

10 coarse wood chips and sandwiching outer layers of finer

wood chips are generally characterised by a high den-
sity. This high density can be ascribed primarily to the
fact that the fibres in the centre layer are positioned
parallel with the longitudinal axes of the chips, i.e.

15 in a plane which extends substantially parallel to the

plane of the manufactured board, and that the chips
during the compression step required to form glue joints
between the chips are compressed to an appreciable
extent such that the density of the board will be sub-

20 stantially higher than the intrinsic density of the

starting material. In the case of conventional chipboard
manufacture, this increase in density will reach about
50%, such as to obtain a characteristic profile which
corresponds to a given chipboard standard, for instance

25 SIS 234 801.

A high density, however, results in chipboard of lower
moisture stability, primarily thickness swelling.
Furthermore, because of its high density, chipboard is
30 normally considered to be heavy and difficult to handle.

The manufacture of conventional chipboard also involves
high costs for starting materials, i.e. chips, and glue,
in addition to energy costs.

WO 89/08539

PCT/SE89/00130

10

15

20

25

30

35

Against the background of the afore-described known
technxgue, there is a desire for a method by means of
which the density of chipboard can be reduced, and
therewith a reduction in the consumption of starting
materials, while retaining the same characteristic
profile in general, or while improving said profile.

A method which touches partially upon this problem is
described in SE-B-346 945. it is stated in this document
(page 10, line 27 to page 22, line 27) that the size of
the chips, the direction of chip fibres and the posi-
tioning or orientation of the chips in the board in-
fluences the properties of the finished chipboard, for
instance such properties as density, swelling, mechani-
cal strength, etc. For example, it has been found that
when the longitudinal axes of the chip fibres are lo-
cated transversely to the plane of the board, the board
will be less compressible as a whole after being formed
and the smaller particles located in the outer layers
will be compressed to a greater extent than the par-
ticles located in the core o*. the board. For the purpose
of obtaining chipboard in which a greater percentage of
fibres are located with the longidunal axes
of the fibres extending transversely to or at an angle
to the plane of the board, it has been proposed in
accordance with SE-B-346 945 to use very short, essen-
tially cubic chips as conventional wood chips, which
because of their short lengths can be positioned with
the long axes, of the fibres extending both vertically
and horizontally and in positions there between. There
xs obtained in this way random orientation of fibres in
all directions.

For the purpose of reducing density and consumption of
starting materials still further, while retaining the

WO 89/08539

PCT/SE89/00130

10

15

20

25

30

35

conventional, useful properties of the finished chip-
board, or even improving the level of such properties,
there is proposed . in accordance with the invention a
method which will impart a more refined and positive
orientation of the chip fibres in the centre layer in a
direction perpendicular to the plane of the finished
board. To this end, it is proposed in accordance with
the present invention that the chips used for the centre
layer of the board are end-grain cut, flake-like chips
whose fibres are oriented in the thickness direction of
said chips, the chips fibres in the centre layer of the
board being orientated substantially transversely to the
plane of the board. By flake-shaped is meant here the
shape of a body whose width and length are substantially
greater than the thickness of the body. When using such
flake-shaped chips, the chips will settle naturally on
their respective base or top surfaces when forming the
centre layer, the longitudinal axis of the fibres being
oriented essentially transversely to the plane of the
board. Due to the substantially unitary transverse
direction of the fibres in the centre layer, a very high
resistance to compression will be encountered in said
layer during the compression stage, and consequently the
conventional type chip particles which form the outer
layers and whose fibres are orientated in the plane of
the board are compressed to relatively thin layers of
high density, whereas the volumetrically larger centre
layer obtains a relatively low density. Tests have shown
that when practicing the method proposed in accordance
with the invention, there can be obtained three-ply
chipboard whose total density is lower than the density
of conventional chipboard, while retaining or improving
the intrinsic or fundamental characteristic profile of
the board. Consequently, chipboard manufactured in
accordance with the novel method will require a smaller

WO 89/08539

PCT/SE89/00130

10

15

quantity of wood starting materials than conventional
chipboard manufactures. This lower consumption of wood
starting materials also decreases costs for glue and
energy in the manufacture of said board. The resultant
high mass surface density of the outer layers provides
denser surfaces, which, for instance, decrease paint and
varnish consumption when treating the surfaces of the
board and enable the board to be lined with thinner
paper liners.

The invention also relates to chipboard, or particle
board, manufactured in accordance with the method and
comprising a centre layer of coarse chips and outer
layers of chips whose fibres are oriented in the plane
of the board, the chips in the centre layer comprising
end-grain cut, flake-shaped chips whose fibres are
oriented in the thickness direction of the chips, the
chip fibres in the centre layer being oriented substan-
tially transversely to the plane of the board.

The invention will now be described in more detail with
reference to the accompanying drawings, in which
Figure 1 is a schematic, longitudinal sectional view of
conventional three-ply chipboard with the chip fibres
extending substantially parallel to the plane of the
board ;

Figure 2 is a schematic, longitudinal sectional view of
three-ply chipboard manufactured in accordance with the
present invention, with the chip fibres of the centre
layer oriented substantially at right angles to the
plane of the board; and

Figure 3 is a schematic plan view of apparatus for
cutting end-grain chips intended for forming the centre
layer chips of chipboard constructed in accordance with
35 the invention.

20

25

30

WO 89/08539 PCT/SE89/00130

The conventional three-ply chipboard illustrated in
Figure l consists -of a centre layer 10 comprised of
chips 12, the fibres of which are oriented in planes
5 which extend substantially parallel with the plane of

the finished board, as illustrated by lines 14. Two
outer layers 16 contain finer chips, so as to provide a
finer and denser surface structure subsequent to being
pressed, the density of the outer layers being higher
10 than the density of the centre layer. Board manufactured

in this way will generally have a relatively high total
density, resulting in heavy board, while the particular
orientation of the fibres in the centre layer renders
the board sensitive to moisture, which is manifested
primarily in swelling of the board in the direction of
its thickness. One reason for the high density of the
centre layer is that the chips with fibres extending
parallel to the plane of the board can be readily com-
pressed, which means that a large quantity of wood
starting material must be used in order to produce
chipboard of given thickness and given acceptable fun-
damental characteristic profile.

15

20

The inventive chipboard, illustrated schematically in
section in Figure 2, consists of a centre layer 18 and
two outer layers 20. In the case of the inventive chip-
board, however, the centre layer 18 is composed of end-
grain cut, flake-shaped chips 22, the fibres of which
are oriented in the thickness direction of the chips, as
illustrated by the lines 24.

As indicated above, the invention is based on the con-
cept of utilizing the inherent resistance to compression
of the wood chips , in a manner to achieve a reduction in
the density of the finished board. This presumes an

WO 89/08539

PCT/SE89/00130

10

15

20

25

30

alternative method of producing the chips and of posi-
tioning the chips in the centre layer of the board.
Thus, there is required a convertible starting material
such as round wood, slabs and edgings, in order to
obtain a unitary chip fibre direction essentially trans-
versely to the plane of the board when forming the
centre layer, the chips 22 need to be end-grain cut
chips which are so configured that, during forming of
the centre layer, the chips will position themselves
such that the largest dimension of the chips will lie
parallel with the plane of the board. It has been found
in practice that flake-like chips or disk-shat>ed Chios
are extremely well suited for this purpose.

Figure 3 illustrates schematically an apparatus for
producing end-grain cut chips suitable for use in the
centre layer 18. Reference is made below to this app-
aratus in conjunction with a description of tests car-
ried out on a laboratory scale in a comparison study
between, on one hand, reference chipboard of conven-
tional composition and manufacture, and, on the other
hand, chipboard manufactured in accordance with the
inventive method, this chipboard having a centre layer
composed of flake-like end-grain cut chips and embraced
by more dense, compressed outer layers.

Tests

An assortment of industrially produced chips were used
as chip material in reference board and for the outer
layers in end-grain board. The centre layer chips of the
reference boards were knife cut with the fibre direction
parallel with the longitudinal axis of the chips, the
chips having a maximum length of about 30 mm.

The chips had an estimated maximum thickness of 2 mm.
The outer chip layer comprised fine chips which fell
within the fraction-composition used in the manufacture
of furniture board having fine-chip outer layers.

The end-grain chips were produced from sawn, undried
spruce planks measuring 65 x 155 mm.

Chip manufacture

In the manufacture of end-grain cut chips for the centre
layer of the inventive chipboard, the planks 25 (Pigure
3) were cut to a length of 90 mm measured in the fibre
direction. Chip cutting was effected with the aid of a
rotating disk 28 having a diameter of 815 mm and provi-
ded with 8 knives (not shown) on one end side. The disk
rotated at a speed of about 900 r.p.m. The following
tool angles were measured: Rake angle r = 45°, edge
angle B = 35°, relief angle a = 10°. The knife setting,
i.e. the distance between the flat disk and the knife
edge, was selected at 1.0 mm. This setting corresponds
to a nominal chip thickness of about l.o mm. In the
manufacture of chips, the planks 26 were placed in a
transport chute with the year rings of the planks facing
towards the knife-carrying disk 28. The planks 26 were
advanced by means of a driven press wheel 3 0 which urged
the planks against the disk. The resultant end-grain
chips thus produced were then dried and fractionated by
passing the chips through a flat laboratory screen
provided with square-mesh wire screen inserts. The
result is set forth in Table 1 below.

1

10

15

20

25

W ° 89/08539 PCT/SE89/OOI30

Iafelfi_i. Fractional composition of the end-grain cut
chips

Fraction Screen dimensions Percentage

— Cjqul) r% , >

i
ii
in

> 8 -° 5.8

" 1 '° 88.5
< 1.0 5,7

The fractions I and III were excluded in the subsequent
board manufacturing process. Thus, solely fraction II
was used, m this respect, the chips in fraction I can
be made smaller and the chips in fraction III can be
incorporated with the assortment of chips for producing
outer layers in the industrial manufacture of chipboard.

Chipboard mannf^f,^

Three-ply chipboard was manufactured at a nominal thick-
ness of 20 mm and a density of 600 kg/m\ The centre
layer constituted 60% of the thickness of the chipboard,
whereas the outer layers constituted 40% of said thick-
ness. For the purpose of studying the influence of layer
density on the characteristic profile of the board, four
density regions were selected for the centre and outer
layers respectively.

30 Table 2 below discloses information concerning the

nominal and measured layer density, the measured layer
thicknesses in mm, the calculated layer distribution and
chipboard designations. Reference chipboard was manufac-
tured solely from conventional industrial chip assort-

35 ments, and is referenced R.

WO 89/08539

PCT/SE89/00130

9

Table 2. Nominal and measured density of surface and
centre layers, and layer thicknesses.

10

15

20

25

Board

Nominal

Measured

Measured layer

Distribut

No.

density

density

thickness

Surface

Centre

Surface

Centre

Surface

Centre

Surf ace-

layer

layer

layer

layer

layer

layer

Centre la

(kg/m J )

■a

( kg/m J )

(kg/m- 3 )

(kg/m -3 )

(mm)

(mm)

(%)

1

825

450

960

400

3.3

13.0

34/66

1R

825

450

780

480

4.1

11.0

43/57

2

750

500

960

410

2.9

13.8

30/70

2R

750

500

790

450

4.0

11.3

41/S';

3

675

550

980

400

2.6

14.5

27/73

3R

675

550

780

460

3.8 —

11.6

40/60

4

600

600

910

430

2.4

14 .8

24/76

4R

600 ■

600

810

500

2.6

14 .2

27/73

The difference between desired nominal board density and
the measured density will be clearly seen from the
30 table.

It will also be seen from Table 2 that board manufac-
tured from end-grain centre-layer chips in accordance
with the invention achieves the desired surface dis-
35 tribution only at a low centre layer density (board 1).

WO 89/08539

PCT/SE89/00130

10

10

15

20

25

30

35

All reference board manufactured from industrially used
chip assortments, -with the exception of board 4R,
achieve the desired layer distribution 40/60.

The method in which the end-grain cut chips intended for
the centre layer (i.e. the fibre direction) and the
geometry of said chips (flake-shaped) are considered to
have contributed to increased compression already in the
low density regions.

The boards were hand-formed, sheet for sheet, in a
forming box measuring 300 x 300 mm. Pressing was
effected under high pressure in a hot press at a
temperature of 180'c. Press plates and spacer strips
were used in the pressing operation. The press closing
time, i.e. the time lapse between upper press-plate
contact and spacer strip contact, was very short in the
case of the reference boards, more specifically an
average time lapse of io seconds. Corresponding boards
having centre layers comprising end-grain chips en-
gendered a compression resistance which resulted in a
compression time of 30-40 seconds.

The influence of press closing time and compression
resistance on layer thickness and layer density can be
seen from Table 3 below, which discloses the density
factor. By density factor is meant here the ratio of the
surface layer density to the centre layer density. The
factor is given both for the nominal density values and
for the measured values. The table also includes com-
pression, i.e. the ratio of measured and nominal factors
(increase and decrease in layer density). The table
shows a marked increase in compression with increased
centre layer density of board manufactured from end-
grain, centre-layer chips in accordance with the inv-

WO 89/08539

PCT/SE89/00130

11

ention. On the other hand, reference boards show a
decrease (1R) and a small increase (2R and 3R) in com-
pression. Board 4R having the highest centre-layer
density also exhibits the greatest increase in com-
pression (Table 3), which corresponds substantially to
that of board 2 which has a lower centre-layer density
of about 100 kg/m3 (Table 2).

10

15

Table 3. Density factors and compression.

Board
No.

1

1R

Density factor

Nominal

Measured

1.83
1.83

2. 40
1.63

Compression
Increase (+)
Decrease (-)
tiu)

+ 31
- 11

20

2

2R
3

3R

1.50
1.50

1^3
1.23

2. 34
1.76

2 . 45
1.70

+
+

+
+

56
17

99
38

4 1 2.12 + 112

25 4R l i. 62 +62

Testing of inherent pr operties

The mechanical strength properties of the boards were
tested in accordance with Swedish chipboard standards
(SIS 234801). Four sample bodies were taken from each
board, for the purpose of determining the bending
strength of the board. Two test bodies were then taken
from the aforesaid test bodies and tested for transver-
sal tensile strength, each body being placed around the

WO 89/08539

PCT/SE89/00130

12

fracture location.

The test carried out on the dimensional stability of the
boards was restricted to investigating swelling of the
boards in the direction of their thicknesses, subsequent
to being submerged in water for 2 hours and 24 hours
respectively. Ten test bodies from each board were
included in this test. It can be mentioned that all
boards had been rubbed down with an abrasive prior to
being tested. The prevailing density of each test body
was also determined.

Result?

The results obtained when testing the intrinsic proper-
ties of the board are set forth in Table 4 below. The
table shows^the measured characteristic properties as a
mean value x with associated standard deviations a for
each individual chipboard. Within each density range
(combination of surface density and centre layer den-
sity) chipboard comprising end-grain centre layers was
compared with reference chipboard whose centre layers
comprised conventional industrial chips.

WO 89/08539

PCT/SE89/00130

13

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WO 89/08539

PCT/SE89/00130

14

Transverse tensile s£Esng£h

The mechanical strength of chipboard is density depen-
5 dent- In the case of chipboard having a conventional

centre layer structure, i.e. a structure in which the
chip fibres are positioned parallel with the plane of
the board, the density of the centre layer is a
critereon of the transverse tensile strength of the
10 board. Test bodies drawn from the transverse tensile

strength test exhibit centre layer fractures, thus also
confirming the strength influencing function of -he
centre layer. A closer study of the results obtained
with the transverse tensile strength test will show that
board whose centre layers comprise end-grain cut chips
have a lower centre-layer density throughout in com-
parison with corresponding reference board, irrespective
of the board type (1-4) measured in accordance with
Table 2. This reduction in density is calculated as
being 10-20%. The reduction in centre layer density
resulted in an increase in the outer layer density of
end-grain cut board by 12-25%.

15

20

25

30

The tests showed that with a board density of 600 kg/m\
the end-grain boards had a transverse tensile strength
of 0.55-0.70 MPa. Corresponding values for conventional
reference boards were 0.45-0.50 MPa. The end-grain
boards had these last mentioned values at densities as
low as 525 kg/m s . Thus, the greater transverse tensile
strength of end-grain board can be utilized in decreas-
ing the board density. The extent of this reduction,
however, is limited by the lowest permitted strength
values .

35

WO 89/08539

PCT/SE89/00130

15

Bendin g strength

Distinct from the centre-layer-density dependency of the
transverse tensile strength, the bending strength of
board is highly dependent on the density of the outer
layers. As earlier established, the surface layer den-
sity of end-grain cut board is 12-25% higher than the
surface layer density of reference board. This also
implies higher strength values in the case of bending or
flexural loads. In the case of end-grain cut board
having a density of 600 kg/m 3 , the bending strength is
from 13-22 MPa, depending on layer density distribution.
Corresponding values for reference board (R-board) is
17-18.5 MPa. In the case of the end-grain cut board 1
and 2 , these reference board values were achieved at
densities as low as 550 kg/m 3 .

Depending on the lowest permitted bending strength
values, the board density of end-grain cut boards can be
made about 50 kg/m 3 lower than conventionally manufac-
tured board, while still achieving the same bending
strength .

Thickness swelling

The centre layer density, and therewith compression,
influences the behaviour of chipboard in the presence of
moisture. For instance, thickness swelling of board will
increase with increased density, which as a rule has
been produced by greater compression. Various methods
are available for restricting swelling, at least with
respect to short-term swelling (storage in water for
less than 2 hours). No swelling inhibiting methods were
applied during the present investigation, however.
Consequently, the absolute swelling values recorded can

WO 89/08539

PCT/SE89/00130

16

be understood as being very high.

Prior to carrying out the swelling tests, the moisture
quotients of the test bodies were measured, wherewith it
was found that the test bodies taken from end-grain
board had a board moisture quotient of 4.0%. Correspond-
ing values for the reference boards were 4.9%.

Table 4 shows the swelling values with associated centre
layer density. In the case of end-grain boards 1-4,
swelling was measured after storing the test bodies in
water for 2 hours, and was found to be on average 13.9%.
Corresponding average values for reference boards 1R-4 R "
were found to be 17.5%. The difference between the
swelling of end-grain board and reference board after
being stored for 24 hours in water are practically non-
existent. The higher centre layer density of the end-
grain boards 3 and 4 engender higher swelling after 24
hours than the boards 1 and 2. The relationship between
density and swelling in the case of the reference boards
is less distinct, with respect to swelling (2 and 24
hours), the end-grain boards 1 and 2 offer advantages
over the corresponding reference boards 1R and 2R.
consequently, a composition according to types 1 and 2
should be chosen for conceivable industrial manufacture.

The afore-described tests carried out on mutually dif-
ferent kinds of centre-layer chips illustrates that the
characteristic profile of three-ply chipboard can be
influenced by the construction of the centre layer, it
can therewith be established that end-grain chips im-
prove such properties as transverse tensile strength,
bending strength and thickness swelling in a marked
manner within a given board density range compared with
conventionally manufactured chips, in which the chip

WO 89/08539

PCT/SE89/00130

17

fibres are oriented in the plane of the board. This fact
can be utilized to .manufacture board of improved charac-
teristic profile and/or in reducing the total board
density. A reduction in board density can assist gener-
5 ally in achieving a reduction in costs, inter alia with

respect to wood, glue, energy, transportation, etc. Low
weight chipboard is desirable to the user of such board,
and such low weight chipboard can be produced in accor-
dance with the invention, as defined in the following
10 claims.

WO 89/08539

PCT/SE89/00130

10

18
Claims

1. A method for manufacturing chipboard comprising a
centre layer (18) of coarse chips and surroundig outer
layers (20) of chips whose fibres are oriented in the
plane of the board, characterised by using as chips for
the centre layer (18) end-grain cut, flake-like chips
(22) whose fibres are oriented in the thickness direc-
tion (24) of the chips, the chip fibres in the centre
layer (18) being oriented substantially transversely to
the plane of the board.

15

20

2. Chipboard comprising a centre layer (18) of coarse
chips and outer layers (20) of chips whose fibres are
oriented in the plane of the board, characterised in
that the chips (22) in the centre layer (18) consist of
end-grain cut, flake-like chips whose fibres are
oriented in the thickness direction (24) of the chips,
the chip fibres in the centre layer (18) being oriented
substantially transversely to the plane of the board.

WO 89/08539 PCT/SE89/00130

7

INTERNATIONAL SEARCH REPORT

Inttmatlonal Application No PCT/SE89 /OOl^O

1. CLASSIFJCATION OF 3USJECT MATTER (if severa! classification svmools aooly, indicate ail) • "

According to international Patent Classification (IPC) or to both National Claasification and IPC 4

3 2? N 3/02, B 32 B 21/02

II. FIELDS SEARCHED '

Minimum Documentation Searched 7

Classification Svt'.em i

Classification Symbols

IPC 4 B 27 N; B 29 C; B 32 B

US CI 425

Documentation Saarchad othar than Minimum Documentation
to the Extant that such Documents ara Included in the Fields Searched •

SE, NO, DK, FI classes as above

111. DOCUMENTS CONSIDERED TO *E RELEVANT*

Category • | Citation ot Document, " with Indication, where appropriate, of the relevant passages i»

Relevant to Claim No.

1

A m \ SE, A, 3^6 9^5 CAB KARLSTADPLATTAN)
I 24 July 1972

See page 10, line 27 -
j page 11, line 27.

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1

1

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1, 2

* Special categories o1 cited documents: «

"A** document defining the general state ot the art which is not
considered to be of particular relevance

"E" earlier document but published on or after the International
filing dale

"L" document wnich may throw doubts on priority clalm<s) or
which is cttad to establish the publication date of another
citation or othar special reason (as specified)

"O" document referring to an oral disclosure, use, exhibition or
other means

"P" document published prior to tha international filing date but
later than the priority date claimed

*T" later document published after the International filing date
or priority data and not in conflict with tha application but
cited to understand the principle or theory underlying the
invention

"X* document of particular relevance; the claimed invention
cannot be considered novel or cannot be considered to
involve an inventive step

M Y H document of particular relevance; the claimed Invention
cannot be considered to involve an Inventive step whan the
document is combined with one or more other such docu-
ments, such combination being obvioua to a person skilled
In the crt.

***■" document member of the same patent family

IV. CERTIFICATION

Date of the Actual Completion of the International Search

1989-06-07

Date of Mailing of this International Search Report

1389 -08- 1 4

International Searching Authority

Swedish Patent Office

(Sj&Saturiof Authorized Officer

Olov irensen

Form PCT/IS A/210 ( second sheet) (Janusry 1*65)

WO PCT/SE89/00130

y